AVCapture中实现拉近拉远镜头

自己用avcapture实现自定制相机，系统相机是可以拉近拉远镜头的，网上搜寻了半天始终没有发现，刚才又翻看了半天的api接口，终于找到了，原来他藏在AVCaptureConnection中，videoScaleAndCropFactor：缩放裁剪系数。简单写了下使用方法：

AVCaptureStillImageOutput* output = (AVCaptureStillImageOutput*)[self.captureSession.outputs objectAtIndex:0];
AVCaptureConnection *videoConnection = [output connectionWithMediaType:AVMediaTypeVideo];
CGFloat maxScale = videoConnection.videoMaxScaleAndCropFactor;
CGFloat zoom = maxScale / 50;
if (zoom < 1.0f || zoom > maxScale)
{
return;
}
videoConnection.videoScaleAndCropFactor += zoom;
self.preVideoView.transform = CGAffineTransformScale(self.preVideoView.transform, zoom, zoom);

其中preVideoView的layer中添加了AVCaptureVideoPreviewLayer。

有一点需要注意：the videoScaleAndCropFactor property may be set to a value in the range of 1.0 to videoMaxScaleAndCropFactor，videoScaleAndCropFactor这个属性取值范围是1.0-[b]videoMaxScaleAndCropFactor，如果你设置超出范围会崩溃哦！[/b]